Another key benefit of the FCC's definitional flexibility and the MB-ODFM approach is much greater latitude for implementing cost-effective UWB hardware/software solutions. As shown in Figure 1, the 3.1 GHz to 4.8 GHz frequency band is ideal for supporting three sub-bands of 500 MHz each. By focusing on the spectrum from 3.1 GHz to 4.8 GHz, the WiMedia specification simplifies the design of the radio and analog front-end circuitry, making it possible to implement complete UWB solutions that are based on existing CMOS process geometries.

When the FCC initially allocated unlicensed spectrum for UWB in 2002 but hadn't yet adopted its final definition, a competition arose between advocates of a single-channel, impulse radio-type approach and the advocates of multiband approaches. Before the industry ultimately settled on WiMedia as the preferred standard, some developers began moving ahead with UWB architectures using exotic compound III-V processes such as silicon germanium (SiGe) or gallium arsenide (GaAs). This allowed them to keep options open for analog-intensive support of single-channel services but it also set them on a path that is constrained by the inherent limitations of III-V processes.

After the Wireless USB promoters group selected WiMedia in 2004 for Certified Wireless USB, Ecma International followed suit in 2005 and the WiMedia specifications were adopted by Bluetooth and European Telecommunications Standards Institute (ETSI) in 2006. This effectively put an end to the single-channel approach and opened the door for cost-effective all-CMOS implementations, however, developers that had already opted for using SiGe or GaAs in their designs were still left facing the limitations of those early design choices.

As it has been frequently demonstrated with previous technology introductions, such as Bluetooth, USB, Wi-Fi, etc., the ability to move rapidly toward mainstream CMOS-based single-chip solutions offers a significant advantage for system designers throughout the market ramp-up and maturation phases. In contrast, designers that start out with multichip implementations and/or exotic processes like SiGe, BiCMOS or GaAs tend to face a significant and often difficult transition at some point in order to bring down their costs and reduce design complexity. From a development efficiency and production ramp-up standpoint, it makes good sense to start as close as possible to where you need to end up.

Standardization on all-CMOS solutions that integrate the complete MAC and PHY functionality on a single-chip enables designers to reduce overall bill of material (BOM) costs, improve reliability and minimize power consumption in the near-term, while leveraging proven CMOS semiconductor processes to optimize cost/performance throughout the market ramp-up and maturation phases.

Use of a single-chip approach is a critical factor for achieving overall power goals. Even CMOS-based multichip implementations have the disadvantage of having to “waste” power and on-chip real estate for implementing the external buses and drivers needed to move data between the chips. A single-chip CMOS-based design that combines the MAC/PHY radio along with other elements of the wireless interface offers a much more compact and power efficient alternative compared to multichip approaches.

Using advanced system-in-package (SiP) techniques such as LTCC, the single-chip CMOS device can be efficiently integrated within the same package with passives, filters, crystals, and other elements of the wireless interface. The block diagram in Figure 2 shows all of the elements that are combined within the SiP package. External interfaces include pin-compatible wired USB 2.0, general-purpose I/O, LCD and optional interfaces to serial Flash memory or EEPROM. A 32-bit RISC processor and on-chip memory are linked via the internal system bus to the MAC interface, which interfaces through the digital baseband to the signal-processing (ADC/DAC) and transmit/receive functions. UWB filter and crystal functions are implemented in the LTCC substrate external to the CMOS chip but contained within the SiP package. The result is a self-contained, fully functional Certified Wireless USB transceiver, which only requires interfacing to an external antenna.

By reducing the cost and complexity of the UWB interface and minimizing external circuitry, an all-CMOS single-chip in a SiP package, such as shown in Figure 3, will help bring overall BOM costs for a complete Certified Wireless USB interface down to the $5 range. This rapid movement down the cost curve will be critical for supporting high-volume deployment of UWB into mainstream wireless applications.

The single-chip all-CMOS approach also takes maximum advantages of WiMedia's design for low-power operation by supporting very fast transitions from active to standby and to active, in a matter of just a few microseconds. CMOS' inherent power efficiency combined with the UWB radio function's ability to move data quickly when it is on, means that the transceiver can spend more time cycled down to the off state in which it draws little power. For example, when using a 480 Mbps radio to transfer 10 Mbps of data, it can operate at a very low duty cycle and be asleep and conserving energy most of the time.

A comparison of various wireless and wired transmission methodologies shows that WiMedia-based UWB is nearly three times more power efficient than Wi-Fi 802.11, when appropriate duty cycles are taken into account. This enables the new generation of Certified Wireless USB devices to deliver significantly better Mb/mW performance than any other wireless standard.

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